Usually, acrylic carbon fiber or graphite fiber is produced by heating acrylonitrile fiber strands in an oxidative atmosphere to produce preoxidized fiber, carbonizing the preoxidized fiber in an inert atmosphere to produce carbon fiber, and finally graphitizing the carbon fiber at a high temperature to produce graphite fiber (as disclosed in, for example, U.S. Pat. Nos. 4,069,297 and 4,197,279).
This process, however, involves several technical problems in the case of continuous operation. That is to say, in the step of carbonization or graphitization, fluff and waste fiber accumulate in the oven to narrow the yarn guides and roller guides. This causes the fiber strand passing through them to fluff. In the step for producing preoxidized fiber from acrylonitrile fiber, a certain degree of coalescence of fibers (sticking of fiber to fiber) is inevitable. Excessive coalescence results in carbon fiber of low strength. In the process for heat treatment of preoxidized fiber, the fiber passes over many rollers and roller guides. They inevitably cause some damage to the surface of the fiber. This surface damage decreases the strength of the carbon fiber. The same applies to the process of graphitization.